A gas plasma is an ionized gas that is capable of conducting electrical energy. Plasmas are used in surgical devices to conduct electrosurgical energy to a patient. The plasma conducts the energy by providing a pathway of low electrical resistance. The electrosurgical energy will follow this path and can therefore be used to cut, coagulate, desiccate, or fulgurate blood or tissue of the patient. One of the advantages of this procedure is that no physical contact is required between an electrode and the tissue being treated.
A plasma is created by ionizing a gas. Some electrosurgical systems have a source of regulated ionizable gas which is directed in a steady flow toward a patient. A gas that is typically used in this manner is argon, however other gasses can also be used. One advantage of having a directed flow of gas is that the plasma arc can be accurately focused and directed by the flow.
Electrosurgical systems that do not incorporate a source of regulated gas can ionize the ambient air between the active electrode and the patient. The plasma that is thereby created will conduct the electrosurgical energy to the patient, although the plasma arc will typically appear more spatially dispersed compared with systems that have a regulated flow of ionizable gas.
One of the difficulties in using a plasma is its initiation. A strong electrical field is required to accelerate enough free electrons within the gas such that a cascade of ionizing collisions is initiated which creates the plasma. This is sometimes called "igniting" the plasma. Once a plasma is ignited, it may be sustained at lower electrical field potentials.
Several techniques are presently used to create strong electrical fields that can ignite the plasma. One technique is to move the tip of an electrode very close to the surgical site. The electric field along a path between an electrode and the surgical site increases as their separation decreases, and may reach a level sufficient to ignite the plasma. The drawback of this method is that a surgeon must carefully manipulate the electrode to move it close to the surgical site without actually touching the tissue. If the electrode comes in contact with the tissue it may stick, causing eschar to deposit on the electrode. During laparoscopic procedures, it is often difficult for a surgeon to sense the proximity of the electrode to the tissue.
Another technique to ignite plasma is to use a pointed electrode which will generate a stronger electrical field at the tip of the electrode. However, a pointed electrode may be undesirable if the surgeon requires a blade-shaped electrode for cutting and other tissue manipulation. Yet another technique is to provide high voltage spikes to the surgical electrode until a detector has indicated a closed circuit with the return electrode. Once a closed circuit is detected, the high voltage spikes are terminated and the electrosurgical generator returns to its normal waveform output. While this technique is effective, it requires complicated electronics and components capable of withstanding the high voltages.
U.S. Pat. No. 4,060,088 relates to an electrosurgical method and apparatus for coagulation by fulguration. The apparatus has a source of inert ionizable gas which surrounds a tubular electrosurgical electrode. A source of periodic bursts of electrosurgical energy is disclosed which is used to initiate the plasma. Also disclosed is an auxiliary discharge to aid in the initiation of the plasma, where the auxiliary discharge results from the electric field established between the doctor's finger and the active electrode.
U.S. Pat. No. 4,781,175 has an ionizable gas jet to clear bodily fluids and coagulate or achieve fulguration in the form of an improved eschar. Circuitry and computer logic are shown to control the gas jet flow and the electrosurgical energy.
U.S. Pat. No. 4,901,720 and the reissue thereof Re. U.S. Pat. No. 34,432 deal with the rate of burst energy pulses, applied to maintain leakage current within acceptable limits while having sufficient energy to initiate ionization. Circuit and logic diagrams are provided to control the burst energy by pulse width, resonance, waveform and output.
U.S. Pat. No. 4,040,426 has a method and apparatus for initiating a plasma by the use of charged particles which are generated as the inert gas flows along a tube. This charge bleeds off of the tube through inwardly pointed tips on the tube, and is used to aid in the initiation of the plasma.
U.S. Pat. No. 4,901,719 has an electrosurgical unit in combination with an ionizable gas delivery system, where there is also an improvement relating to the gas conducting means.
The disclosures of the aforesaid references are incorporated by reference and made a part hereof.
The present state of the art has not completely overcome the difficulties associated with ignition of a plasma in surgical systems. A more simple and reliable apparatus and method for igniting a plasma will make it possible for plasma-based systems, and hence surgeons, to work more effectively.